Are indicators of stress and pain recognizable during lip twitch in horses? A behavioral investigation

Lip twitching is a physical restraint method for horses that may function by inducing short-term stress and/or pain. The present study aimed to: 1) evaluate whether behavioral indicators of pain and stress could be recognized during the application of a lip twitch in horses across different experimental conditions involving electrical, mechanical and thermal stimulation; 2) explore the relationship between observed variables and individual personality traits. Twelve horses were initially included in a two-sessions twitch trial. This study focused on data collected during the first session, which was completed by ten horses out of twelve. Behavioral analysis was performed on 2-minute videos recorded during three-time intervals (Baseline, Twitch and Post-Twitch) applying an ethogram. Personality was evaluated using a validated questionnaire completed by the horses' caretakers. Any interval, condition and personality effect on the behavior was analyzed using a GLMM. Results showed that maintenance of a natural head position decreased significantly during Twitch and Post-Twitch (p<0.001), while unequal forelimb weight distribution increased during Twitch and Post-Twitch (p<0.001) compared to Baseline. A significant effect of interval was found for the duration of ears forward (p = 0.026), with shorter durations recorded during Twitch compared to both Baseline and Post-Twitch. Furthermore, ears movement decreased (p<0.001) and head movements increased (p<0.001) during Twitch. Horses’ responses to twitch varied based on personality: neurotic horses displayed rather passive stress signs (prolonged ear-backward, p=0.032), while agreeable horses showed more active avoidance (moving away, p=0.073). These findings suggest that in horses tolerating the application of the twitch, the restraint induces mild stress, with individual coping mechanisms influenced by personality. Thus, it appears important to consider individual characteristics when applying the technique in a clinical context. This approach is essential to ensuring the safety of both horses and personnel during veterinary interventions, especially in the absence of pharmacological restraint or training. The data supporting the findings of this study are available upon reasonable request from the corresponding author. • Lip twitching may function by activating endogenous modulation in horses. • Twitching may help avoid or reduce the need for sedatives and tranquilizers. • Some horses exhibit higher stress and aversion to the twitch compared to others. • Mild stress can be observed in horses that tolerate the procedure. • Personality traits appear to influence behavioral responses to twitching.